Nowadays, the field of use of modern laser scanner systems is widespread. In particular, they are used to determine distances to other objects, for obstruction identification, for object identification and for recording of a three-dimensional image of the three-dimensional surrounding area. This is particularly relevant for safety-critical applications, for example for controlling unmanned aircraft (UAV—Unmanned Aerial Vehicle), in which visually assisted perception of the surrounding world is required).
Laser scanner systems, which operate both in the 2D range and in the 3D range, are generally based on one of two techniques. In the first variant, a laser light source is used whose light is scattered in the surrounding area via mechanically moved mirrors and/or joints. For example, a two-dimensional scanning process is provided by using a mirror which is moved by a mechanical drive, or a mechanical drive which moves the laser source. Two drives are required to produce a three-dimensional scan.
In the second variant, a plurality of light sources and/or lasers are mounted on one axis, as a result of which, in principle, there is no need to scan or scatter the laser light for a two-dimensional scan. On the one hand, this has the major advantage that it increases the scanning rate, but it has the critical disadvantage that the technology is more complex, expensive and large, and this has a negative effect, in particular in the field of unmanned aircraft. In order to obtain a three-dimensional image, it is also possible to scan the laser beam emission source, which is mounted on the axis, along the perpendicular to this axis, thus leading to a so-called “fan beam”.
In addition to these three-dimensional laser scanner systems which are based on more than one light source and/or laser source, it is also possible to upgrade the widely used 2D laser line scanners, which are based on only a single light source, by additional mirrors or joints, thus allowing three-dimensional scan images to be produced. In this case, the use of 2D laser line scanners to produce a 3D scan image has the critical advantage that, because of the light weight and the acceptable size as well as the relatively low costs in comparison to pure 3D laser scanners, the fields of application are widened considerably, thus allowing systems such as these also to be used, for example, for perception of the surrounding area in an unmanned aircraft (UAV). By way of example, “Whalley, N:; et al.: Design and flight test results for a hemispherical ladar developed to support unmanned rotorcraft urban operations research, Aeroflightdynamics Directorate (AMRDEC) US Army Research, Development & Engineering Command Ames Research Center, San Jose State Foundation Ames Research Center, Perot Systems Government Services Ames Research Center, Feb. 2008” describes an unmanned helicopter which could carry out a three-dimensional scan by means of a rotating 2D laser line scanner.
In principle, there are two variants to extend the two-dimensional emission area into the third dimension in order to obtain a three-dimensional scan from a 2D laser line scanner, In the first variant, as is described in “Wulf, O.; Wagner, B.: Fast 3d scanning methods for laser measurement systems in International Conference on Control Systems and Computer Science, volume 14 Jul. 2003”, the entire laser head or the laser beam emission apparatus is moved such that the laser beam is emitted in a three-dimensional scan area. By way of example, this makes it possible to suspend the laser head on the x-axis of the laser beam such that it carries out a joint movement and thus transforms the two-dimensional line scan to the third dimension (pitching scan, variant A). Another option is to continuously rotate the laser or the laser beam emission apparatus, thus resulting in a hemispherical or spherical 3D scan area (rolling scan, variant B). In the case of a so-called rolling scan, the 3D scan area is in this case dependent on the axis on which the laser head is suspended such that it can rotate.
The second variant for obtaining a 3D scan from a 2D laser line scanner is in this case to allow the emitted laser beams to be reflected on a polygonally rotating polygonal mirror, such that the 2D line scan is transformed to the third dimension, A combination such as this of a 2D laser line scanner and a rotating polygonal mirror is correspondingly described in “T.C. Ng. Development of a 3D Ladar system for autonomous vehicle guidance, Technical report SIMTech, 2005”. The major advantage of the second variant is in this case that only the polygonal mirror is rotated, which makes up a fraction of the weight of the entire laser scanner, thus resulting in very much reduced torques being produced, which is actually a critical factor for use in small UAVs. This is in fact the major disadvantage of the first variant, in which the complete line scanner is pitched and/or rotated, leading to corresponding forces and torques because of this movement of the scanner, Particularly in the case of small UAVs, this can lead to problematic flight behavior. A further disadvantage of the first variant and of the so-called rolling scan is in this case that no cable connection is used between the actual line scanner and the evaluation unit, which means that the required data must be transmitted via sliprings. Particularly at relatively high rotation speeds, this can lead to contact problems in the sliprings, and therefore to loss of data, which is a criterion for exclusion in the field of safety-critical applications.
A further disadvantage of the second variant, in which a 2D laser line scanner is combined with a rotating polygonal mirror, is that only a certain proportion of the emitted laser beams can be used for the 3D scan since, because of the rigid form of the polygonal mirror, a not inconsiderable proportion of the emitted laser beams are transmitted such that they do not strike the polygonal mirror. These laser beams are then no longer available to the scanner.
However, all variants have the considerable disadvantage in comparison to a stereo camera that the total scan time per 3D image or per 3D scan is relatively long. For example, a stereo camera requires about 0.04 second per recording, while a rotating 2D line scanner requires around 2.39 seconds per 3D scan. The so-called pitching scan configuration still results in 1.2 seconds, while around 0.8 seconds are still required per 3D scan for the variant with the polygonal mirror (for a chosen resolution of 1°). If the resolution is matched to a maximum of 0.25°, then the time required per scan is in this case multiplied by sixteen times. The following table once again shows the comparison in its totality.
TABLEResolutionResolutionResolution1°0.25°0.16°Laser scanner2.3 s38.33 s —-rotating-(scan area 180° × 180°)Laser scanner1.2 s19.2 s—-pitching-(scan area 180° × 90°)Laser scanner0.8 s12.8 s—-polygonal mirror-(scan area 90° × 60°)Stereo camera——0.04 s(field of view 50° × 40°)
The measured data is based on the SICK LMS 201 laser scanner and a digital Firewire camera which operates with a framerate of 25 fps and a resolution of 320×240 pixels.
Particularly in safety-critical areas, for example in the UAV area, the required scan times of laser scanners are too long to ensure appropriate real-time operation. In this case, the scan times can approach those of a stereo camera only at the expense of angular resolution, or vice versa.